Radiation is all around us, every minute of every day. The harnessing of radiation is one of the most significant achievements of the past century. The use of radiation enables one with such power that it tends to extract the extremes of human behavior, both good and bad. The ability to monitor and control radiation levels in each instance of its use is of concern to the entirety of humankind.
Radiative particles can be characterized by energy, mass, and charge. The detection of a radiative particle requires the presence of a medium that is sensitive to the energy, mass, and/or charge of the particle of interest. The medium must be held under conditions of strict constraint so that any reaction can be uniquely attributed to an encounter with the radiative particle.
Gamma rays are massless, chargeless high energy particles. Because they are massless and chargeless, the appropriate medium is one that is sensitive to the high energy of the gamma ray. A gas, liquid gas or liquid semiconductor fulfills such a need.
The atomic structure of the medium must be characterized by an energy level architecture that can be disturbed in a measurable way by the high energies resulting from a gamma ray encounter. The nature of disturbance is such that one or more atomic electrons are dislodged upon each encounter in a process called ionization. A single gamma will produce many such electrons, the number of which is directly proportional the original gamma energy.
Once electrons have become ionized, they must be counted in order to deduce the gamma ray energy. This usually means the electron must be subjected to the force of an electric field that pulls it to a collection surface called the anode. However, one must be aware of the fact that it is possible for the electron to be reabsorbed by the gas before arrival at the collection surface.
The favored geometry for a gas, gas-filled or liquid semiconductor detector is a closed coaxial cylindrical volume. A potential difference exists between the inner and outer cylindrical surfaces, the former of which can be simply a wire collinear with the axis of the cylindrical structure. Most often the inner surface, the anode, is held at positive potential while the outer surface, the cathode, is grounded. A medium known to be reactive to the energy of the radiation of interest is confined at within the volume between the cylindrical surfaces.
Incoming radiation penetrates the outer cylindrical wall and interacts with the active substance. Such interactions result in the dislodging of electrons within the atomic structure of the semiconductor. The freed electrons are then subjected to the force of the electric field resulting from the potential difference between the inner and outer cylindrical structures. Electrons are attracted to the inner surface while positively charged residual entities are attracted to the outer surface. The total charge collected, usually only on the inner surface, is proportional to the energy of the incoming radiation.
In order for such a signal to be effectively detected, all the charge from one event must be swept from the collection area before the next event occurs. The detector's capacitance, i.e., its tendency to store charge, is an impediment to this process. Because the capacitance is directly proportional to its length, the efficient performance of a gas-filled detector is highly impacted by the length of the cylindrical assembly.
On the other hand, many molecules must be available for interactions to occur. This dictates the need for high density and/or detection volume of the active medium. Detection volume is directly proportional to length of the cylindrical assembly and to the square of its radius. Accordingly, the need to minimize the detection length in order to preserve low capacitance is in direct opposition to the need for maximizing it in order to provide detection volume.
Detection volume can also be increased by an increase in radius. However, an increased radial dimension results in an increased path length that a freed electron must travel in order to get to the positively charged inner surface and be counted. The longer the path length that the freed electron must travel, the more likely it is to be reabsorbed by the gas. If the electron is reabsorbed, it is not counted.
Even if the above considerations are well balanced, the signal resulting from charge collection competes with inherent sources of noise in the circuit, most notably that due to the so-called “series current”. A lower detector capacitance enables the signal to overcome the noise. Such considerations elucidate the need for a detection system having a volume dimensioned in such a way that it does not retain a high capacitance, while still providing a sufficient probability of capturing incoming radiation without reabsorbing it.
In the past, such problems have been solved by simply segmenting the detector system. An array of single detectors satisfies both concerns. The total detection volume is the sum of the array segments while the capacitance is limited to the capacitance of only one segment. However, the resulting duplicity in electronic components results in significantly increased cost and maintenance. Moreover, an array of prior art radiation detectors possesses a significant amount of “dead space”, simply due to the dimensional requirements of each packaged detector element and its peripheral components. Consequently, detector arrays are only a partial solution to the problem.
It is an objective of the present invention to provide a detector with increased detection volume, resolution, and efficiency, without increased detector capacitance.
It is an objective of the present invention to provide a detector to be used as an array element, wherein the use of the many such detectors enables maximum packing density while minimizing the cost of ancillary components.
The above objectives are met by enclosing multiple detection units within a single external structure forming a large segmented detector wherein each segment has good resolution but the share common elements. In the exemplified embodiment, two detectors arranged in tandem share a common cathode structure and pressure vessel but have separate anode structures and signal leads. This effectively doubles the detection volume without doubling the capacitance and yet still avoids undue duplication of components. The yoking of multiple units in this manner allows sharing of a filling port, high voltage power supply, pressure relief, and pressure vessel. Because the latter must generally meet the Department of Transportation Regulations (for example (CFR 49)) significant cost savings are enjoyed by avoidance of its duplication.